Somebody's alternate personality or persona; another self.
A very close and intimate friend.
A corporation used by a person to conduct personal business in an attempt to shield himself or herself from personal liability, and which a court may penetrate by "piercing the corporate veil" to impose liability on the person when they commit fraud or injustice.
An assistant to another person, especially to a superior or more important person.
In literature, theatre/theater, etc., a good foil of the protagonist, a character who helps emphasize the traits of the main character.